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For pt.II, see ibid., vol.7, no.6, 730 (1974). The apparatus and experimental procedure for optical electric resonance investigation of a fast hydrogen beam are described. Particular attention is paid to the design and performance of the radiofrequency system. The generalized rotating field model of the atom-RF field interaction developed in Part I and the optical detection function derived in Part II are employed in conjunction with computer simulation and least squares fitting techniques to extract information on the initial states from experimental optical electric resonance spectra of the H(n=4) manifold.

Measurements of the inclusive pion electroproduction reaction e − +p → e − + π ± +X made at the Wilson Synchrotron Laboratory are reported. Data were taken at points with nominal ( Q 2 , W ) values of (0.6, 2.67), (1.2, 2.67), (2.0, 2.67) and (1.2, 2.15) in (GeV 2 , GeV). A more complete angular distribution was taken at (1.2, 2.67). The virtual photoproduction cross section for π + + missing mass is studied as a function of missing mass, Feynman scaling variables, Q 2 and W . The invariant cross section show a simple behavior as a function of Q 2 and W . Its dependenc on P ⊥ 2 , x ′ and φ is determined.

Measurements of the electroproduction reaction ${e}^{\ensuremath{-}}+p\ensuremath{\rightarrow}{e}^{\ensuremath{-}}+{\ensuremath{\pi}}^{+}+n$ carried out at the Wilson Synchrotron Laboratory are reported. For fixed virtual photon-hadron center-of-mass energy, $W=2.67$ GeV, data are presented with the mass of the virtual photon, $\ensuremath{-}{k}^{2}$, centered at 0.6, 1.2, and 2.0 Ge${\mathrm{V}}^{2}$. At the central $\ensuremath{-}{k}^{2}=1.2$ ${(\mathrm{GeV})}^{2}$ point, results are also given for $W=2.15$ GeV. For the $W=2.67$, $\ensuremath{-}{k}^{2}=1.2$ Ge${\mathrm{V}}^{2}$ setting, an angular scan is presented and the longitudinal transverse-interference term is extracted. The data are compared with an electron Born-model calculation of Berends which has as its only free parameter the pion electromagnetic form factor. The theory is used to extract new results for the pion form factor up to $\ensuremath{-}{k}^{2}=2.0$ Ge${\mathrm{V}}^{2}$.

This series of articles will describe a novel method of utilizing optical detection of RF-induced fine structure transitions to investigate the population of excited states of fast hydrogen atoms produced by charge exchange collisions between an initial proton beam and various gas or foil targets. In Part I a model of the charge exchange collision and the interaction of the hydrogen beam with an external oscillatory field are considered within the framework of the density matrix formalism and expressions are obtained for the RF lineshapes in terms of the initial atomic density matrix elements.

The lineshape theory of optically detected RF-induced electric resonance transitions in hydrogen is concluded with the treatment of the detection process within the framework of the density matrix formalism. An optical detection function is derived from which can be determined the intensity, polarisation, and angular distribution of the detected decay radiation, subject to constraints of apparatus efficiency and geometry, in terms of the initial atomic density matrix elements and variables of the radiofrequency electric field.

We report the first observation of the atomic cascade $9^{1}P_{1}\ensuremath{-}7^{3}S_{1}\ensuremath{-}6^{3}P_{1}$ in mercury. The exponential distribution of time delays between detection of the $9^{1}P_{1}\ensuremath{-}7^{3}S_{1}$ (5676-\AA{}) photon and the $7^{3}S_{1}\ensuremath{-}6^{3}P_{1}$ (4358-\AA{}) photon was measured, and a value of 8.2(0.2) nsec was obtained for the mean lifetime of the $7^{3}S_{1}$ level.

We have observed directly the time correlation between successively emitted photons in the $7^{3}S_{1}\ensuremath{-}6^{3}P_{1}\ensuremath{-}6^{1}S_{0}$ cascade of atomic mercury, and have deduced from it direct measurements of both the natural lifetime of the $6^{3}P_{1}$ state and the dependence of the mean correlation time (coherence time) on the density of the mercury vapor surrounding the primary radiation region. The density-dependent coherence time constitutes a direct observation of the imprisonment of resonance radiation and provides an experimental confirmation of the theory describing this phenomenon.

This paper reports studies of the optical absorption spectrum and the $X$-band electron spin resonance spectrum of rubidium atoms trapped in an argon matrix. The samples were formed by deposition from a mixed beam of the two constituents onto a cold plate. The optical absorption measurements showed two groups of lines. The red group, whose absorption lines are near those of the free atom, has a triplet structure; the blue group, whose absorption lines are shifted roughly 1650 ${\mathrm{cm}}^{\ensuremath{-}1}$ to the blue, has four lines. The ESR measurements showed three major trapping sites. A motorized circular polarizer was used to measure the differential absorption of right and left circularly polarized light. By monitoring the differential absorption of right and left circularly polarized light in the presence of a microwave field, a correlation was found between the optical absorption sites and the microwave absorption sites. A crystal-field model is employed in an attempt to fit the data and to understand the nature of the sites.

We report measurements of the electroproduction reaction ${e}^{\ensuremath{-}}+p\ensuremath{\rightarrow}{e}^{\ensuremath{-}}+{\ensuremath{\pi}}^{+}+n$ carried out at the Cambridge Electron Accelerator by observing the final electron and pion in coincidence. The apparatus consisted of two focusing magnetic spectrometers equipped with wire spark chambers for determining the trajectories of the particles and with \ifmmode \check{C}\else \v{C}\fi{}erenkov counters for particle identification. Data were collected in three scans which varied in turn one of the three variables: the photon mass squared ${k}^{2}$, the virtual photon-hadron center-of-mass energy $W$, or the angle $\ensuremath{\theta}$ between the virtual photon and the electroproduced pion in the virtual-photon-hadron center-of-mass system. The ${k}^{ 2}$-scan central settings were ${k}^{ 2}=\ensuremath{-}0.18,\ensuremath{-}0.30,\ensuremath{-}0.40,\ensuremath{-}0.80$, and -1.20 Ge${\mathrm{V}}^{2}$ for $W=2.15$ GeV and $\ensuremath{\theta}={0}^{\ensuremath{\circ}}$; the $W$-scan central settings were $W=1.85,2.05,2.15$, and 2.50 GeV for ${k}^{ 2}=\ensuremath{-}0.30$ Ge${\mathrm{V}}^{2}$ and $\ensuremath{\theta}={0}^{\ensuremath{\circ}}$; the central settings for the $\ensuremath{\theta}$ scan were $\ensuremath{\theta}={0}^{\ensuremath{\circ}},{7}^{\ensuremath{\circ}}$, and 15\ifmmode^\circ\else\textdegree\fi{} for $\ensuremath{\varphi}={0}^{\ensuremath{\circ}}$ and 180\ifmmode^\circ\else\textdegree\fi{} and for ${k}^{ 2}=\ensuremath{-}0.40$ Ge${\mathrm{V}}^{2}$ and $W=2.15$ GeV. The data are analyzed to determine the longitudinal-transverse interference term and are compared with the predictions of a dispersion-theory calculation by Berends. The dispersion-theory model is used to extract the pion electromagnetic form factor. The result is consistent with ${F}_{\ensuremath{\pi}}={F}_{1}^{V}$, but a simple $\ensuremath{\rho}$-pole form factor cannot be ruled out.

Measurements are reported on the successful orientation of 26-hour ${\mathrm{As}}^{76}$ donors in a doped silicon crystal. A general discussion is given of the various methods of dynamic nuclear orientation and how they apply to ${\mathrm{As}}^{76}$. The use of radio-frequency orientation to determine the sign of the nuclear magnetic moment and the mixture of total angular momenta carried off by the electron-neutrino system is discussed. ${\mathrm{As}}^{76}$ has been oriented by both saturation of forbidden transitions and double resonance methods. The hyperfine splitting of ${\mathrm{As}}^{75}$ in the doped silicon crystal is +198.42\ifmmode\pm\else\textpm\fi{}0.04 Mc/sec; the hyperfine splitting of ${\mathrm{As}}^{76}$ in the same crystal is -93.660\ifmmode\pm\else\textpm\fi{}0.06 Mc/sec. The spin of ${\mathrm{As}}^{76}$ has been shown to be 2; its gyromagnetic ratio, -0.4514. The determination of the relative amounts of angular momenta carried off by the ${2}^{\ensuremath{-}}$\ensuremath{\rightarrow}${2}^{+}$, 2.41-Mev beta ray gives a result which is consistent with 5 parts $L=0, 2$ parts $L=1, \mathrm{and} 3$ parts $L=2$. An unknown systematic error in these last measurements makes it difficult to say precisely what the mixture is. Unsuccessful attempts to produce orientation through the Overhauser and Abragam methods are summarized.

The hyperfine structure of atomic nitrogen in the ${(2p)}^{3}^{4}S_{\frac{3}{2}}$ electronic ground state has been investigated by the spin-exchange optical polarization method. The transmission of circularly polarized rubidium ${D}_{1}(5^{2}P_{\frac{1}{2}}\ensuremath{\leftrightarrow}5^{2}S_{\frac{1}{2}})$ resonance radiation through a flask containing rubidium vapor, a buffer gas, and atomic nitrogen was observed as a function of the frequency of an applied radio-frequency field. Measurements were made at pressures ranging from 10 to 65 mm Hg in neon, argon, helium, and molecular nitrogen buffer gases. The extrapolated zero-pressure frequency intervals and their corresponding pressure shifts, as determined by the method of least squares, are

This paper describes an apparatus for cooling an iron foil containing a radioactive impurity to a temperature in the range of 0.012 to 0.026°K by thermal contact with an adiabatically demagnetized chromium potassium alum salt pill. The purpose of the demagnetization is to orient the nuclei of the radioactive impurity. A superconducting solenoid is used to produce the high magnetic field required to remove the entropy from the chrome alum. The apparatus uses two helium Dewars—a large one which contains the superconducting magnet and a small one which contains the salt pill, the 1°K helium bath, and the radioactive sample. The salt pill is formed by pressing a mixture of chrome alum and Octoil S around 5000 0.076‐mm‐diam copper wires. The salt is supported by a graphite cylinder and helium exchange gas is used to establish heat contact between the salt and the 1°K helium bath. In one form of the apparatus the chrome alum salt and the sample are surrounded by a copper can which is cooled to 0.1°K by contact ...

We report measurements of the photoproduction from hydrogen of single ${\ensuremath{\pi}}^{+}$ mesons at gamma-ray energies of 3.4 and 5.0 BeV and at laboratory angles of 5.1\ifmmode^\circ\else\textdegree\fi{}, 7.1\ifmmode^\circ\else\textdegree\fi{}, 9.9\ifmmode^\circ\else\textdegree\fi{}, and 15.1\ifmmode^\circ\else\textdegree\fi{}. The $s$ dependence at fixed $t$ is derived for momentum transfers of -0.20, -0.37, and -0.70 Be${\mathrm{V}}^{2}$. The pion data are compared with a Reggeized one-pion-exchange model.

The cross section for scattering of electrons off carbon has been measured at laboratory angles of 3.3\ifmmode^\circ\else\textdegree\fi{}, 6\ifmmode^\circ\else\textdegree\fi{}, and 12\ifmmode^\circ\else\textdegree\fi{} for incident energies of 1 to 5 GeV. The measured cross sections are in good agreement with the sum rules describing this process.

The spin-exchange optical pumping technique has been used to measure the gyromagnetic ratios of hydrogen, tritium, and free electrons in terms of the electronic gyromagnetic ratio of ${\mathrm{Rb}}^{85}$. A direct measurement was also made of the ratio of the electronic $g$ factors of tritium and hydrogen. The 60-G magnetic field in which these measurements were made was produced by a precision-wound solenoid which was surrounded by three concentric magnetic shields. The magnetic field varied by 5 parts in ${10}^{6}$ over the region occupied by a typical absorption flask. The free electrons were produced by ionization resulting from the beta decay of the radioactive tritium atoms. Measurements were made in various sizes of absorption flasks and in flasks containing different pressures of helium, argon, and neon buffer gases. The measured $g$-factor ratio did not depend upon the nature or pressure of the buffer gas. The results of these measurements were as follows: $\frac{{g}_{J}(\mathrm{Rb})}{g(e)}=1+(6.3\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}; \frac{{g}_{J}(\mathrm{Rb})}{{g}_{J}(\mathrm{H})}=1+(23.74\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6};$ $\frac{{g}_{J}(\mathrm{T})}{{g}_{J}(\mathrm{H})}=1\ensuremath{-}(0.11\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}; \frac{{g}_{J}(\mathrm{H})}{g(e)}=1\ensuremath{-}(17.4\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}.$

The optical transmission of an optically oriented rubidium vapor in spin-exchange equilibrium with atomic hydrogen, deuterium, and tritium has been used to measure with high precision the hyperfine splittings of these paramagnetic atoms. The results are $\ensuremath{\Delta}\ensuremath{\nu}(\mathrm{H})=1420.405726\ifmmode\pm\else\textpm\fi{}0.000030 \mathrm{Mc}/sec,$ $\ensuremath{\Delta}\ensuremath{\nu}(\mathrm{D})=327.384349\ifmmode\pm\else\textpm\fi{}0.000005 \mathrm{Mc}/sec,$ and $\ensuremath{\Delta}\ensuremath{\nu}(\mathrm{T})=1516.701396\ifmmode\pm\else\textpm\fi{}0.000030 \mathrm{Mc}/sec.$ These results are based on a value of the hyperfine splitting of ${\mathrm{Cs}}^{133}$ which is taken to be $\ensuremath{\Delta}\ensuremath{\nu}({\mathrm{Cs}}^{133})=9192.631840 \mathrm{Mc}/sec.$These measurements were made in various buffer gases which caused a shift in the observed hyperfine splitting, and the results given represent extrapolations to zero pressure. The pressure shifts were measured for H in argon, neon, helium, and molecular hydrogen and were measured for D and T in argon and neon. The assigned limits of error represent the range of disagreement of the zero-pressure extrapolations in the different buffer gases.

An electronic computer has been used to investigate the six nuclear matrix elements which enter into the ${2}^{\ensuremath{-}}$ to ${2}^{+}$ 1.40-MeV beta transition in the decay of ${\mathrm{Sb}}^{122}$. Data from beta-gamma angular correlation, beta-circularly polarized gamma angular correlation, nuclear orientation, and nuclear resonance experiments were used in this analysis. As a further aid, the Feenberg-Ahrens relations between certain of the nuclear matrix elements were employed to catalog the solutions and to simplify the search problem. In order to discover how the remaining ambiguity of these solutions could most easily be reduced, for each of the solutions calculations were made of the predicted results of all possible experiments on this beta transition. These calculations show how sufficient experimental data can be obtained to determine unambiguously all six nuclear matrix elements. In an appendix all the theoretical formulas which give the experimental observables for a first forbidden ${2}^{\ensuremath{-}}$ to ${2}^{+}$ beta transition in terms of the nuclear matrix elements are summarized.

Measurements of the photoproduction from hydrogen of single K + mesons at gamma ray energies of 3.4 and 5.0 GeV and at laboratory angles of 5.1°, 7.1°, 9.9° and 15.1° are reported. The s dependence at fixed t is derived for momentum transfers of −0.25 and −0.37 (GeV) 2 .

A new technique for the study of the beta rays emitted by polarized nuclei is presented. This technique uses two silicon detectors arranged in coincidence so as to discriminate against gamma rays and back scattering from the detectors. This technique has been used to study the asymmetry of the beta rays in the decay of $^{110m}\mathrm{Ag}$.

A theory is derived to predict the density matrices describing two atoms after a spin-exchange collision from the density matrices before the collision and the scattering amplitude for binary collisions. This theory is then applied to a system of quasifree electrons interacting with optically pumped rubidium atoms. For calculating the change in the electron density matrix, the ${\mathrm{Rb}}^{85}$ and ${\mathrm{Rb}}^{87}$ are replaced by a fictitious rubidium isotope with no nuclear spin. Expressions are derived for the change in the light transmission when a radiofrequency field is present at the electron frequency. In addition to the linewidth for the electron signal, the spin-exchange theory predicts a frequency shift the magnitude of which depends upon the two-body scattering amplitude, the rubidium polarization and the rubidium density. Experiments performed to test various aspects of the theory are then reported. The measurements were made on a system of quasifree electrons interacting with rubidium atoms. Measurements of the electron linewidth as a function of temperature indicated that spin-exchange collisions dominated the electron relaxation. The predicted frequency shift was measured by observing the electron resonance frequency first with left circularly polarized light and then with right circularly polarized light. For electrons colliding with rubidium atoms the shift is negative when the rubidium polarization is positive and the ratio of the shift to the linewidth is $\frac{\ensuremath{\delta}{\ensuremath{\nu}}_{0}}{\ensuremath{\Delta}\ensuremath{\nu}}=\ensuremath{-}0.025\ifmmode\pm\else\textpm\fi{}0.005$. From the measured values of the shift and the linewidth, a value is derived for the electron-rubidium spin-flip cross section. In one Appendix the replacement of the ${\mathrm{Rb}}^{85}$ and ${\mathrm{Rb}}^{87}$ by the fictitious rubidium isotope with no nuclear spin is justified. In a second Appendix the calculations for the simple electron-rubidium system are generalized and applied to more complicated systems. Results are reported for the change in the density matrix of hydrogen atoms when they collide with polarized electrons, the change in the density matrix of ${\mathrm{Rb}}^{87}$ atoms when they collide with polarized electrons, and the change in the density matrix of hydrogen atoms when they collide with hydrogen atoms.

The peripheral model was used to describe the onepion-exchange photoproduction at laboratory angles of 1 to 11 deg at 2 to 6 Bev. Experimental and theoretical photoproduction cross sections are presented. (R.E.U.)

The theory, design, and construction of a magnetically shielded solenoid are described. Three correction coils are employed in addition to the main solenoid winding. The solenoid is enclosed in three concentric magnetic shields which serve to screen the inner region of the solenoid from external magnetic fields. First the theory and design of a solenoid with correction coils and no magnetic shielding are discussed. Then calculations of the magnetic field due to a solenoid inside a closed cylinder of infinite permeability are summarized. These calculations show that a properly constructed shield can improve the homogeneity of the field due to a single solenoid by an order of magnitude. Optical pumping measurements of the field homogeneity in the central region and of the field distribution along the axis of the solenoid are reported. The measurements of the field distribution agree with the calculations to within a few hundredths of a percent. The shields reduce the disturbance due to changing external fields by a factor of 100. The main solenoid is 91.44‐cm long and has an inner diameter of 32.41 cm. The solenoid field is 18.6 G/A and the solenoid dissipates approximately 320 W when producing a field of 60 G. The solenoid‐shield system has at 60 G a homogeneity of 1 part in 105 over an 8‐cm‐diam sphere at the center of the solenoid. Construction details of the shields and coils are given and the solenoid power supply is described.

We have studied the reaction e−+p→e−+π++n by detecting the final electron and pion in coincidence. Data are presented in the region of virtual photon mass squared from -0.18 to -1.2 GeV2, and virtual photoproduction center-of-mass energy and angle from 1.85 to 2.50 GeV and 0 to 20°, respectively.

The Schrodinger equation for the interaction of a decaying two-level atom with an external oscillating field is separated via a series of matrix transformations into a time independent and a time dependent part; the former yields the radiofrequency Stark shift for decaying states, the latter is normally negligible for atomic beam resonance experiments. In the limit of stable states, or for the case of two levels of equal lifetime, the shift reduces to that of the Bloch-Siegert effect. Exact analytical expressions for the state amplitudes are obtained for the case when the time dependent matrix can be neglected; the resulting transition probabilities are investigated as functions of frequency, time, and interaction strength and are compared with the rotating field theory and the exact numerical solutions.

An experiment testing quantum electrodynamics at high energies and small distances is described. The photoproduction from carbon of electron-positron pairs was measured at laboratory angles of 4.60\ifmmode^\circ\else\textdegree\fi{}, 6.23\ifmmode^\circ\else\textdegree\fi{}, and 7.46\ifmmode^\circ\else\textdegree\fi{}. Symmetrical electron-positron pairs in the energy range from 1 to 5 BeV were detected with a magnet-counter system which consisted of two mirror-image arms. Extensive internal checks of the apparatus were made and the results were reproducible. The theoretical values for the electron-pair yield were calculated by integrating the differential pair-production cross section over the acceptance of the apparatus using a Monte Carlo technique. The ratio $R=(\mathrm{experimental}\mathrm{yield})/(\mathrm{theoretical}\mathrm{yield})$ was not 1.0. $R$ was approximately given by $R=0.62{(1.00\ifmmode\pm\else\textpm\fi{}0.05)+\frac{{k}^{2}}{{(4.31\ifmmode\pm\else\textpm\fi{}0.17)}^{2}}}$ where $k$ is the energy in BeV of the photon which produced the pair, and by $R=0.67{(1.00\ifmmode\pm\else\textpm\fi{}0.04)\ensuremath{-}\frac{{{Q}_{f}}^{2}}{{(313\ifmmode\pm\else\textpm\fi{}13)}^{2}}},$ where ${{Q}_{f}}^{2}$ is the four-momentum of the virtual fermion in ${(\mathrm{MeV})}^{2}$. The apparatus studies and a comparison of the measured single-electron yields with the theoretical yields suggest that an error exists in the absolute normalization of the results. There are no indications that the observed variation of the electron-pair yields with momentum or the large excess of wide-angle electron pairs at high energies is due to any systematic error. The experimental results do not agree with the predictions of quantum electrodynamics; they indicate a breakdown of the theory or the presence of other processes.

The spin-exchange optical polarization method was used to remeasure the zero-field hyperfine splittings of atomic hydrogen and atomic tritium. The transmission of circularly polarized rubidium resonance radiation through a flask containing rubidium, atomic hydrogen, or atomic tritium, and a buffer gas was monitored as a function of the frequency of an applied radio-frequency field. Measurements were made for hydrogen in helium, neon, molecular hydrogen, and argon buffer gases and for tritium in neon and argon buffer gases. The values of the hyperflne splittings obtained are DELTA nu (H) = 1,420,405,749.1 plus or minus 6.0 cps; DELTA nu (T) = 1,516,701,476.8 plus or minus 6.0 cps. The assigned limits of error represent the range of disagreement of the zeropressure extrapolations in the different buffer gases. The fractional pressure shifts STApressure shift in cps/mm Hg divided by the hyperfine splitiing in kMc/secl for hydrogen in helium, neon, molecular hydrogen, and argon are 4.80 plus or minus tively. Those for tritium in neon and argon are 3.24 plus or minus 0.03 and - 5.05 plus or minus 0.15, respectively. (auth) Munoz for measuring atomic partial stopping powers ( xi /sub o/) has been extended to the determination of the stopping power xi /submore » 1/ of hydrogen gas for the H/sup +/ component of a hydrogen beam. The protons in a 1-mm dia. beam are held in an orbit which is an arc of a circle of 142-cm radius, and are guided to a 1-mm dia. exit after a path length of 55.8 cm in --50 mu pressure of H/sub 2/ gas. The energy of the beam after leaving the stopping cell is measured by electrostatic deflection. No proton thus measured can have experienced a charge-changing collision in the stopping cell. In preliminary experiments protons were simulated by deuterons of twice the energy and xi /sub 1/ determined for protons of 55, 75, and 93 kev. Finally, an extended series of runs at 42.8 and 54.2 kev were made on xi /sub 1/ , and a similar extended series on xi /sub 0o/, with the apparatus arranged as in the work of Allison et al. These latter measurements for xi /sub 1/, xi , and xi /sub 0/ gave (6.12 plus or minus 0.44; 6.36 plus or minus 0.16; 2.72 plus or minus 0.16) x 10/sup -15/ ev cm/sup 2// atom, and in the same units, for the higher voltage, (6.04 plus or minus tion of the beam is known from other experiments, the charge-changing collisions may be shown to contribute 34% of the total stopping losses. The sum of the losses in kinetic energy in an electron capture and in its subsequent loss is 40 plus or minus 6 ev at42.8 kev and 54 plus or minus 9 ev at 54.2 kev, neglecting charge-changing collisions involving H/sup -/. (auth)« less

The construction and design principles of a focusing atomic beam apparatus are reported. This apparatus, which is of the radio‐frequency magnetic resonance type and which has been used to measure the spins and hyperfine splittings of four radioactive nuclides as reported elsewhere, utilizes the six‐pole focusing magnet of Friedburg and Paul and of Korsunskii and Fogel but in a way which avoids velocity aberrations. From an analysis of the optimization of intensities in the conventional and the focusing types of atomic beam apparatus it is concluded that the latter has an advantage of a factor of roughly twenty‐five.

The optical transmission of an optically oriented sodium vapor in spin-exchange equilibrium with atomic nitrogen has been used to measure the zero-field hyperfine splitting of ${\mathrm{N}}^{14}$ and ${\mathrm{N}}^{15}$. The ground state of atomic nitrogen is $^{4}S_{\frac{3}{2}}$. For ${\mathrm{N}}^{14}$, which has $I=1$, $\ensuremath{\Delta}{\ensuremath{\nu}}_{\frac{5}{2}\ensuremath{\rightarrow}\frac{3}{2}}=26.12721\ifmmode\pm\else\textpm\fi{}0.00018 \mathrm{Mc}/sec,$ $\ensuremath{\Delta}{\ensuremath{\nu}}_{\frac{3}{2}\ensuremath{\rightarrow}\frac{1}{2}}=15.67646\ifmmode\pm\else\textpm\fi{}0.00012 \mathrm{Mc}/sec.$ For ${\mathrm{N}}^{15}$, which has $I=\frac{1}{2}$, $\ensuremath{\Delta}\ensuremath{\nu}=29.29136\ifmmode\pm\else\textpm\fi{}0.00016 \mathrm{Mc}/sec.$

Measurements of the spin-lattice relaxation times of arsenic donors in a doped silicon crystal at 8500 gauss and 1.3\ifmmode^\circ\else\textdegree\fi{}K are reported. The relaxation times for the stable ${\mathrm{As}}^{75}$ were measured by using adiabatic fast-passage techniques to observe the relative amplitudes of the electron resonance signals as a function of time. The relaxation times for the radioactive ${\mathrm{As}}^{76}$ were measured by observing the formation and decay of the nuclear alignment. For the ${\mathrm{As}}^{75}$, of which there are 2.8\ifmmode\times\else\texttimes\fi{}${10}^{16}$ atoms/${\mathrm{cm}}^{3}$, the direct electron relaxation time is 6.5\ifmmode\pm\else\textpm\fi{}0.5 minutes and the relaxation time for simultaneous spin-flips of the nucleus and the electron is 4.7\ifmmode\pm\else\textpm\fi{}1 minutes. For the ${\mathrm{As}}^{76}$, of which there are 3\ifmmode\times\else\texttimes\fi{}${10}^{10}$ atoms/${\mathrm{cm}}^{3}$, the direct electron relaxation time is 4\ifmmode\pm\else\textpm\fi{}2 minutes and the relaxation time for simultaneous spin-flips of the nucleus and the electron is \ensuremath{\ge}75 minutes.

A search was conducted for dipion resonances in the mass range 1360–1780 MeV photoproduced in the forward direction from a carbon target. No evidence for dipion resonances was found.